The disposal of organic sludge presents a serious problem, especially for municipalities having large quantities of sewage sludge to be disposed of. Disposal efforts are costly, involving well-known expedients such as burning the sludge in an incinerator, or hauling the sludge to a distant location for dumping or burial. The high water content of organic sludges is a major difficulty, since wet sludges cannot be burned without costly expenditure of high grade fuels to sustain combustion. Moreover, transporting of the sludge is made much more expensive by the necessity of transporting the large quantity of occluded water.
Analysis of typical raw sewage sludge reveals a water content of 80 to 90% by weight. The remaining solids generally comprise 50 to 70% organic matter typically having the following elemental breakdown: 60% carbon; 8.5% hydrogen, 27.5% oxygen and 4% nitrogen, plus inorganic solid matter including calcium, silicon, and metal values. When the raw sludge has been dewatered to a level of 60 to 70% moisture where it can be burned, it is found to have a colorific value as high as 10,000 BTU per pound, although when burned the useful yield is more likely to be in the range of 5000 to 7000 BTU per pound. This difference is accounted for by the energy required for evaporating the remaining water content. After the sludge has been dewatered to a level of 60 to 70% moisture, the organic content of the partially dewatered sludge can provide sufficient fuel value to sustain autogenous incineration at a temperature of about 1400.degree. F., which is sufficient to dispose of the organic matter, destroy odors and pathogens, and accomplish these purposes without the necessity of adding fuel for firing the incinerator. Thus apparently autogenous combustion of sewage sludge is a highly desirable method of disposal which can also become a source of cheap energy.
The dewatering of sludge has been attempted in many different ways, aside from costly direct combustion or heat treatment of the sludge. One very desirable way of dewatering sludge is to do so under natural conditions, for instance by spreading the sludge on the ground and allowing bacterial digestion to continue while using sunlight or ambient heat for evaporating the moisture as the cellular structure slowly breaks down. However, the time-span involved is usually quite unacceptable, since it may require from two to six months depending on climatic conditions. Natural dewatering is highly dependent upon geographic location and cannot be used efficiently in most places. There have been a number of proposals for hastening a natural digestive process by the use of chemicals, for example as disclosed in U.S. Pat. No. 3,772,188 to Edwards, wherein sewage is treated with doses of oxygen to promote rapid growth of aerobic bacteria to accelerate digestion of the sludge. Another prior art approach is to attempt to mechanically dewater the sludge by rupturing the cellular structure by exerting a rapid pressure drop following application of pressure to physically rupture the cells. Such a process is disclosed in U.S. Pat. No. 4,159,944 to Erickson et al, but it is a highly energy intensive step used prior to incineration. U.S. Pat. No. 4,124,459 to Blanch et al teaches the idea of treating electrolysis brine sludge, not a bacterial organic sludge, with an acid to lower its pH below 2.5, and then leaching it with hypochlorite solvent to recover its mercury content.
In an article from the Journal of the Environmental Engineering Division, December 1977 by Sukenik, King and Olver, pages 1013-1021, entitled "Chlorine and Acid Conditioning of Sludge", the authors describe treating an organic sludge with acid and hypochlorite to increase its filterability. The hypochlorite was introduced in very small quantities, approximately 1 gram per liter of organic sludge, which is about one-tenth the amount used to the present disclosure, and the author acknowledges that the acid rather than the chlorine was mainly responsible for the observed results. This article also discusses the fact that heavy metals were released to the supernatant liquid in such quantity as might seriously impact upon a biological waste treatment system, indicating that not enough hypochlorite was added to raise the pH back up again by release of occluded water prior to separation of the supernatant liquid from the solids so as to precipitate the metal values. The article also recognizes the change in color of the sludge to light tan along with the dewatering and consequent improvement in filterability. The authors further stated that the addition of chlorine treatment sharply decreased filterability of the sludge samples as compared to the application of acid alone. These results do not match the results of the present disclosure probably for the reason that an insufficient oxidation of the sludge resulted in failure to actually break down the organic cells and release the higher pH water from them.
German Pat. No. 2,648,788, dated May 26, 1977 discusses the dewatering of sludge by lowering the pH value to approximately 2.2, and then heating the acidulated sludge to about 65.degree. to 95.degree. C. to break down the organic matter at the expense of a large energy expenditure and excessive release of metal values into the supernatant liquid. Moreover, there is an undesirable dissolving of organic materials in the supernatant liquid as a result of this process.
Finally, there is a Japanese pending patent application which was published as (KOKAI KOHO) #149258/1979 on Nov. 22, 1979, based on pending Japanese application #59198/1978. This publication seeks reduction of the very large quantity of flocculants normally used in the absence of the publication's improvement, i.e. about 3-10% ferrous chloride and about 30% lime which raises the pH of the sludge to about 12.2. The Japanese publication proposes to alter the quantity of flocculants to about 10 to 20% ferrous sulfate and about 10 to 15% lime (by weight of solids) by adding sufficient acid to lower the pH to about 5, and adding hydrogen peroxide in the quantity of about 7 to 9% (by weight). This publication uses about twice as much acid as the present disclosure, perhaps because of the large quantity of lime. Apparently such use of the acid is intended to attack the organic materials, rather than to act merely as catalyst to improve the performance of the oxidizing reagent. The quantity of oxidizing reagent, hydrogen peroxide, used in the publication is also about twice the quantity used in the present disclosure when measured on a molar basis. Selecting the average value by weight of hydrogen peroxide at 8%, the Japanese figure amounts to about 0.235 moles per 100 grams of solids, whereas the present disclosure uses only about 0.116 moles per 100 grams. The Japanese process uses so much additive that the final cake of solids is 50 or 60% foreign material which is not combustible, and therefore the calorific value of the cake is greatly reduced. Moreover, the resulting Japanese ash after incineration is much less satisfactory since the metal values have been diluted so as to reduce the value of the resulting incinerated ash in a metal value recovery process of the type disclosed in U.S. Pat. No. 4,033,763 to Markels.